We have been using the rat hepatic malic enzyme gene as a model to study the mechanism of thyroid hormone action. Herein we have shown that the ME TRE sequence at position -287/-257 of the ME gene 5' flanking region can bind T3 receptor and function as a very potent T3 responsive element either as a repressor of the TK promoter activity (-T3) or an enhancer (+T3). Mutation and deletion analyses of ME TRE revealed that this region contains several important subregions, two of them, separated by 10bp, are essential for the altered transcriptional activity of the TK promoter. We have also been studying the interactions of the thyroid hormone receptor with other transcriptional factors participating in the assembly of transcription initiation complex. Using ME promoter sequence we have identified a transcriptional factor whose binding activity to the region -50 to -70 is altered by thyroid hormone and thus might determine together with the receptor, the frequency of initiation of mRNA synthesis. To study this interaction the relevant transcriptional factor has to be purified. Using liver nuclear extracts fractionated on a DEAE-sepharose column and heparin-agarose we have partly purified two different proteins that bind to the ME sequence -50/-70. Further purification will be accomplished using a DNA affinity column to generate material for microsequencing and antisera production.